Implement rule priority groups to tree_morph

[lixitrixi]

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[lixitrixi]

This is good for now IMO.

I'll make a separate PR where I improve the documentation and naming. One thing Nik mentioned is that I use "transform" and "reduce" interchangeably, which is confusing! People in the VIP also mentioned that reduce implies "making smaller", which isn't necessarily true.

Apparently "morph" means "undergo or cause to undergo a gradual process of transformation." and has similar meaning in computer graphics as "change smoothly from one image to another by small gradual steps using computer animation techniques." which I think is perfect! The below conventions might be good:

• morph: an operation which applies multiple smaller changes gradually, like the 3 main functions
• transform: a function which possibly changes a given node into another, along with side effects
• rule: another word for a "transform" implemented by the user. These are usually simple, matching against a single specific form.
• rule group: combines multiple rules into one. Along with a selector function, this makes a transform!